When flying with the A320 and A321, altitude indicated in the MCDU changes suddenly all alone, often to crazy values like 70000 or 99000 ft. Let’s say I am flying at FL200, and suddenly the MCDU changes from 20000 into a random FL.
I tend to exclude an issue with A32NX (FlyByWire) as the issue is present also when using the A321 (without A32NX). I have not tested thoroughly other aircraft. (FYI I use the 5.2 stable version of A32NX).
This happens in particular if I change viewpoint (say from CTRL 1 (pilot view forward) to CTRL 5 (FMC). The moment I leave the previous view I see the value changing, as if an imaginary copilot had instantly rotated the altitude knob. The randomly appearing new value has nothing to do with the flightplan and, as said, is often completely beyond the max ceiling altitude of the aircraft.
This may happen often during a flight, not at all in the next, and just a few times on another one. The issue tends to be more frequent in the initial flight phase.